The US Senate has narrowly blocked a Republican-led attempt to allow drilling for oil in Alaska's Arctic National Wildlife Refuge (ANWR).
Up to 10bn barrels of oil may lie beneath the Alaskan tundra
Supporters of the plan fell four votes short of the number needed to prevent opponents using a filibuster - or delaying tactic - to derail the vote.
The Republicans had hoped to win Senate support by tacking the Alaska measure on to a major defence spending bill.
Both sets of legislation are likely to be presented again separately.
Supporters of drilling in Alaska say it offers an alternative source of energy to the Middle East and so would improve national security.
Opponents warn oil exploration would harm a pristine wilderness and endanger a key habitat for migratory birds, polar bears, caribou and other animals.
The Republicans fell short of the 60 votes needed to avoid the Democrat-led filibuster, with the tally standing at 56-44.
Senate leaders are expected to withdraw the defence spending bill to redraw it without the Alaska drilling provision.
Alaska's Arctic National Wildlife Refuge covers 19 million acres
Senate Majority Leader Bill Frist was among those to vote 'No' because, under Senate rules, as a member of the majority he would be allowed to bring up the drilling issue for another vote.
Alaska Republican Ted Stevens had attached the measure to the defence spending bill - an essential piece of legislation - in the hope it would be passed.
Fellow Alaska Republican Lisa Murkowski also backed it, saying: "If we can do anything more to reduce our reliance on oil imports, we need to do it."
But some Republicans joined the filibuster for fear the defence bill would be derailed by the ANWR measure.
Democrats accused Senator Stevens of holding the defence bill "hostage" to Alaska drilling.
"We all agree we want money for our troops... This is not about the troops," said Democratic Senator John Kerry.
Democratic leader Harry Reid said the Senate could move quickly to pass the defence bill once the ANWR issue had been resolved.
Opening a portion of the refuge to oil and gas exploration has been a goal of US companies for a quarter of a century - and is a key objective of the Bush administration.
The White House believes access to the estimated 10bn-plus barrels of crude oil would cut American reliance on imports, create jobs and raise revenue.